The invention relates to a direct induction melting device (or furnace) for dielectric substances which are insulating when cold, but which become conducting above a temperature threshold proper to each substance, which will be called temperature of inductibility or of electrical conduction in what follows. More precisely, such substances are perfectly insulating when cold but their resistivity decreases sharply and becomes, for example, less than 10.sup.-2 ohm/m.sup.2 /m, with the increase in temperature, beyond a threshold value called temperature of inductibility, between for example 600.degree. and 1000.degree. C., so that they become capable of conducting currents induced in the mass by an alternating magnetic field (of medium or high frequency) generated by means of an inductor surrounding a charge of such a substance.
In the following text, the expression "direct induction" will mean that the seat of the induced currents is the charge itself, which is to be heated by them up to and/or above its melting point. For a charge comprising solely substances which are insulating when cold of the above-mentioned type, a portion thereof must be preheated so as to reach the temperature of inductibility with conventional means such as a removable "susceptor" made from a refractory material, which is conducting when cold, such as metal (tungsten, molybden, for example) or graphite plunged temporarily into the divided charge (see, for example, publication FR-A-No. 1 509 985, page 2, left-hand column, lines 6 to 16), or, if it is a question of melting metal oxides one of whose metal components reacts in a highly exothermic way with oxygen, i.e. by releasing a great deal of heat, there may be deposited on or in the charge, a small quantity of grains or shavings of this metal which, when molten by induction, oxidize and allow the adjacent parts of the charge to become locally conducting so that they become the seat of induced currents (see, for example, publication FR-A-No. 1 427 905, page 2, right-hand column, lines 6 to 17). For example, when it is a question of melting glass containing silica (SiO.sub.2), it is possible to use metal silicon powder or grains which, when oxidizing, is added to the silica of the charge without contaminating it when oxidized.
In a direct induction electric furnace, no intermediate heating means is used, such as a refractory metal or graphite crucible, between the inductor and the charge. This means that one may use either a crucible made from an insulating refractory material with a temperature of inductibility higher than the melting temperature of the charge (see FR-A-No. 2 054 464), or a cage or a crucible cooled to a temperature substantially lower than the temperature of inductibility of the charge, which crucible may be made from a dielectric material (quartz or silica, see FR-A-No. 1 509 985) or from a conducting material, formed by means of tubular metallic elements which are arranged side by side to form a cylinder and insulated from each other (see for example FR-A-No. 1 492 063 corresponding to U.S. Pat. No. 3,461,215 or GB-A-No. 1,221,909, where the inwardly-turned faces of the cooled walls of the cage (or crucible) are overlaid with a layer formed by the substance constituting the charge, in a powdered, granular or sintered form, or else agglomerated by the heat or a substantially continuous transition between these forms. This peripheral layer which is electrically and thermally insulating, replaces the prefabricated refractory crucible (from whence the name "self crucible" or "in situ" melting pot formation).
In publication FR-A-No. 1,186,996 there is described and in the above-mentioned publication FR-A-No. 1 492 063 (see page 1, left-hand column, lines 17 to 24 and 30 to 38), there is mentioned a direct induction melting device in which an inductor formed by a double metal wall, cooled by internal fluid flow, serves simultaneously as single-turn inductor, crucible and member for shaping an of ingot of electrically molten material. More precisely, the single-turn inductor comprises two coaxial cylindrical metal walls (of different diameters) respectively closed off at their ends by annular walls, as well as by two parallel and adjacent longitudinal walls which form therebetween a narrow longitudinal slit, so as to form a hollow and sealed conducting body which is capable of being cooled by an internal fluid flow (water). The two ends of the single open turn, situated on each side of the slit, are respectively electrically coupled, by means of an impedance matching transformer to the terminals of an AC generator of suitable power.
As in the case of the above-mentioned cold cage, the peripheral part of the charge of the substance which is dielectric when cold, which covers the cooled inner wall of the single turn inductor, forms a cylindrical envelope which insulates this wall electrically and thermally from the rest of the charge which is, molten or which has reached its temperature of inductibility.
In the vicinity of the slit of the single turn inductor there is produced a circumferential electric field of high intensity which has practically no harmful effect, if the materials forming the charge have temperatures of inductibility not very different from their respective melting point (this is the case for silica, magnesia and alumina, for example). Other refractory materials, for example metal oxides such as zirconia, titanium oxide or thoria, or mixtures containing one of these, have temperatures of inductibility substantially lower (1000.degree. to 1500.degree. C.) than their melting points (2000.degree. to 3000.degree. C.), and they may then become conducting in the solid state and contribute to the formation of electric arcs between the edges of the slit which, by heating the adjacent parts, would produce short circuits capable of stopping the melting and/or damaging the inductor and the generator.
The value of the inductance of a cylindrical single-turn inductor formed from a flat strip is an increasing function of the radius of the turn and a decreasing function of the width of the strip, which limits appreciably the height of the volume of the charge. Since this inductance is relatively small, it is necessary to use high or very high frequencies (between a few hundred kilohertz and a few megahertz), for which tube oscillators are indispensable, for thyristor inverters whose efficiencies are very high generally do not exceed 50 kHz. Power triodes and tetrodes, usable in such oscillators, have relatively high anode or internal resistances with respect to the load impedance of the parallel resonant circuit comprising the single turn inductor, which implies the use of an impedance matching transformer whose primary winding in the anode circuit comprises a large number of turns with respect to those of the secondary, as well as a leakage inductance which may absorb a large part of the reactive power developed. The depth of penetration of the currents into the charge as well as stirring of the bath are also lower at high frequencies.
The present invention allows most of the drawbacks of the state of the art known up to now to be remedied, which drawbacks result either from the use of the single turn inductor crucible associated with the use of a high frequency or from the use of a cold multi-segmented cage whose presence in the field of the inductor causes both additional losses by Joule effect in the cage and a coupling loss between the inductor and the charge. This coupling loss reduces, on the one hand, the electric efficiency of the inductor in so far as the heat contribution to the charge is concerned and, on the other hand, it attenuates the effect of the electromagnetic forces confining and stirring the bath, beneficial in the present case. The thickness of the solid film which is formed, in the neighborhood of the cold wall, is thus reduced and consequently the heat insulation which it provides with respect to the charge in contact therewith.